An electronically controlled fuel injection valve is opened by a driving pulse signal (injection pulse) given synchronously with the rotation of an engine and while the valve is opened, a fuel is injected under a predetermined pressure.
Accordingly, the injection quantity of the fuel depends on the period of opening of the valve, that is, the injection pulse width. Assuming that this pulse width is expressed as Ti and is a control signal corresponding to the injection quantity of the fuel, Ti is expressed by the following equations: EQU Ti=Tp.times.COEF.times..alpha.+Ts and Tp=K Q/N
wherein Tp stands for the injection pulse width corresponding to the basic injection quantity of the fuel, which is called "basic fuel injection quantity" for convenience, K stands for a constant, Q stands for the flow quantity of air sucked in the engine, N stands for the rotation speed of the engine, COEF stands for various correction coefficients for cerrecting the quantity of the fuel, which is expressed by the following formula: EQU COEF=1+Ktw+Kas+Kai+Kmr+Kacc+Kdcl
in which Ktw stands for a coefficient for increasing the quantity of the fuel as the water temperature is lower, Kas stands for a correction coefficient for increasing the quantity of the fuel at and after the start of the engine, Kai stands for a correction coefficient for increasing the quantity of the engine after a throttle valve arranged in an intake passage of the engine is opened, Kmr stands for a coefficient for correcting the air fuel mixture, Kacc stands for a correction coefficient for increasing the quantity of the fuel at the time of acceleration of the engine and Kdcl stands for a correction coefficient for decreasing the quantity of the fuel at the time of deceleration of the engine,
.alpha. stands for an air-fuel ratio feedback correction coefficient for the feedback control (.lambda. control), described hereinafter, of the air-fuel ratio of the air-fuel mixture, and Ts stands for the quantity of the voltage correction for correcting the change of the flow quantity of the fuel injected by the fuel injection valve, which is caused by the change of the voltage of a battery.
In short, the desired injection quantity of the fuel is obtained by multiplying the basic fuel injection quantity Tp by various correction coefficients COEF, and when a difference is brought about between the targeted, or aimed at value to be attained by the control and the actual controlled value, this difference is multiplied by .alpha. to effect the feedback control and the correction for the power source voltage is added to the feedback control.
The feedback control of the air-fuel ratio will now be described. An exhaust component concentration detecting member, for example, and O.sub.2 sensor for detecting the oxygen component in the exhaust gas, is attached to an exhaust passage to detect the actual air-fuel ratio .lambda. of the air-fuel mixture sucked in the engine, and by comparing with a slice level, it is judged whether the actual air-fuel ratio .lambda. is richer or leaner than the target air-fuel ratio .lambda.t. When a known ternary catalyst for efficiently convertion CO, HC and NOx, the main three exhaust gas components, at the theoretical air-fuel ratio is arranged in the exhaust system, the above-mentioned aimed at air-fuel ratio .lambda.t is equal to the theoretical air-fuel ratio. Accordingly, in this case, by the slice level, it is judged whether the actual air-fuel ratio is richer or leaner than the theoretical air-fuel ratio, and the injection fuel quantity expressed by Tp.times.COEF is increased or decreased and controlled so that the actual air-fuel ratio becomes equal to the theoretical air-fuel ratio. For this control, the air-fuel ratio feedback correction coefficient .alpha. is set and the injection quantity Tp.times.COEF is multiplied by .alpha..
If it is intended to effect the feedback correction at a time by abruptly changing the value of the air-fuel feedback correction coefficient .alpha., the theoretical air-fuel ratio is overshot or undershot, and therefore, the value of the air-fuel ratio feedback correction coefficient is changed by the proportion and integration (PI) control so that the air-fuel ratio is stably controlled.
More specifically, in the case where the output of the O.sub.2 sensor is higher or lower than the slice level, the air-fuel ratio is not abruptly leaned or riched, but in the case where the air-fuel ratio is rich (lean), the air-fuel ratio is first decreased (increased) only by the proportional (P) component, and is then gradually decreased (increased) by the integration (I) component unit so that the air-fuel ratio is leaned (riched). The P component is set at a value sufficiently larger than the I component unit.
In the region where the air-fuel ratio feedback control is not performed, the value of .alpha. is clamped to 1 or a constant value.
Needless to say, if the base air-fuel ratio in the region where the air-fuel ratio feedback control is effected, that is, the air-fuel ratio at the time when .alpha. is equal to 1, is set at the theoretical air-fuel ratio (.alpha.=1) through the entire region, the feedback control is inherently unnecessary. Practically, however, even if the base air-fuel ratio is set at .lambda.=1 in a specific driving state, the air-fuel ratio is ordinarily deviated from the theoretical air-fuel ratio in other driving states because of deviations or changes with the lapse of time among constituent members (such as an air flow meter, a fuel injection valve, a pressure regulator and a control unit), the non-linearity of the pulse width-flow amount characteristic of the fuel injection valve and changes of the driving conditions and environments. In this region where the deviation of the base air-fuel ratio occurs, the air-fuel ratio feedback control is performed so that this deviation is eliminated. This air-fuel ratio feedback correction control is disclosed in, for example, U.S. Pat. No. 4,284,050.
However, in this air-fuel ratio feedback control, for example, when one stationary driving region is greatly changed to a different stationary driving region, the base air-fuel ratio in this different stationary driving region is greatly deviated from .lambda.=1 and it takes too long a time to perform the PI control of the change of the base air-fuel ratio generated by this deviation to .lambda.=1 by the feedback control. More specifically, even though the base air-fuel ratio has been obtained from the specific injection quantity Tp.times.COEF and the deviation of this air-fuel ratio from the theoretical air-fuel ratio has been corrected by the PI control based on .alpha., since the base air-fuel ratio is greatly changed, the base air-fuel ratio is controlled to a value greatly different from .lambda.=1 if Tp.times.COEF used up to this time is still used, and the feedback correction by similar PI control should be performed and it takes a long time to correct the base air-fuel ratio to .lambda.=1 by the feedback correction. In order to eliminate this disadvantage, it is necessary to improve the respondency of the control by increasing the PI constant. However, if the control respondency is thus improved, overshooting or undershooting is readily caused and the control performance is degraded. Namely, when the base air-fuel ratio is greatly deviated from .lambda.=1, the control of the air-fuel ratio is effected in the region separate greatly from the theoretical air-fuel ratio.
Consequently, the driving is carried out in the range where the conversion efficiency of the ternary catalyst is low, and therefore, increase of the cost by increase of the amount of the noble metal in the catalyst is caused and the catalyst should be exchanged with new one frequently because of further reduction of the conversion efficiency due to deterioration of the catalyst.
The acceleration correction coefficient Kacc and deceleration correction coefficient Kdcl of COEF involve problems similar to those described above with respect to .alpha.. As taught in, for example, U.S. Pat. No. 3,483,851 and U.S. Pat. No. 3,750,632, the fuel injection quantity is increased at the time of acceleration of the engine and the fuel injection quantity is decreased at the time of deceleration of the engine. However, in a so-called single point injection system where one fuel injection valve is arranged in an engine, for example, upstream of a throttle valve, at the time of acceleration of the engine, a part of the fuel increased for acceleration adheres to the wall of the intake passage and a response delay is caused in substantial increase of the injection quantity of the fuel for acceleration. Furthermore, at the time of deceleration of the engine, since the fuel adhering to the wall of the intake passage is sucked in the engine, even if the injection quantity of the fuel is decreased, the quantity of the fuel sucked in the engine is not promptly decreased. Accordingly, the respondency is reduced at the time of acceleration or deceleration of the engine. This increase or decrease of the fuel at the time of acceleration or deceleration has a considerable influence on the fuel ratio at the transient driving, and hence, the above-mentioned air-fuel ratio feedback control is adversely affected. Accordingly, at the time of acceleration or deceleration, .alpha. is clamped to 1 or a certain value to stop the air-fuel ratio feedback control and so-called feedforward control of obtaining a fuel injection quantity corresponding to the base air-fuel ratio by multiplying Tp by Kacc or Kdcl is performed. However, if the same Kacc or Kdcl is always used, because of deviations of constituent members of the engine, changes with the lapse of time and environmental changes, necessary increase or decrease of the fuel quantity for acceleration or deceleration is not performed when a great change is produced in the air-fuel ratio according to the driving state.